The present invention relates to transfer of waste heat from exhaust gas to incoming combustion air in a combustion furnace. Preferred embodiments also achieve reduction of particulate emissions. The invention has particular utility in the melting of glass, but is applicable to any large-scale combustion furnace.
Commercial production of glass in a continuous melting furnace conventionally involves feeding raw glass batch materials into an opening at one end of an elongated melting furnace while withdrawing melted glass through an opening at the opposite end of the furnace and forming it into the desired product, such as a flat glass ribbon. Flat glass batches typically include sand (silica), soda ash (sodium carbonate), limestone (calcium carbonate), dolomite (calcium carbonate and magnesium carbonate), rouge (iron oxide), a source of sulfur such as salt cake, gypsum, slag, etc. as a fining agent, and sometimes the raw materials aplite, feldspar, or nepheline syenite. It is also known to use caustic soda in place of soda ash. Minor amounts of additional materials such as colorants (e.g., iron oxide) may sometimes be used as well. These batch ingredients, in finely divided, dry, particulate form, are blended together and usually wetted with water (or caustic soda solution) prior to being introduced into the furnace. Additionally, a substantial amount of cullet (broken or crushed glass) is mixed with the batch ingredients, in amounts usually ranging from about 20% to about 60% of the total glassmaking materials being fed to the furnace.
In the melting furnace, the major source of thermal energy for melting the raw ingredients into molten glass is usually the combustion of fossil fuel (natural gas or fuel oil) in flames above the melting mass. Exhaust gases leave the melting chamber at very high temperatures, typically on the order of 1400.degree. C. to 1700.degree. C. Much of the heat in the exhaust gases may be recovered in regenerators or recuperators, but typically a substantial amount of thermal energy is wasted. It is not uncommon to find exhaust gases being discharged to a smokestack at about 300.degree. C. to 800.degree. C. Recovery of waste heat by conventional heat exchange technology has been found to be generally economically unfavorable, and has been hampered by the corrosiveness of glass furnace exhaust and the presence of particulate emissions that tend to clog heat exchange means.
The use of sulfur compounds in the melting process is a major contributor to particulate emissions from a glass melting furnace. At glass melting temperatures, sulfur compounds such as salt cake dissociate or volatilize, resulting in the emission of sulfur-containing gases which pass from the furnace in the exhaust gas stream. A portion of these sulfurous gases recombine with sodium vapor within the furnace or exhaust passages to form particulate sodium sulfate which comprises the major portion of the particulate emissions from glass furnaces, particularly flat glass furnaces. In some localities, regulations as to maximum allowable particulate emission rates have restricted operations of some glass manufacturers. Many widely varying proposals for reducing particulate emissions from glass furnaces have been made in the prior art, but none is entirely satisfactory.
Perhaps the most straightforward approach to lowering the amount of particulates in glass furnace emissions is to treat the exhaust gas stream to remove the particulates such as by electrostatic precipitators or scrubbers. However, such approaches entail high capital and operating costs and return no improvement to the melting process. Accordingly, it has been suggested that the exhaust gas stream contact glass batch materials to strip particulates from the exhaust as well as preheat the batch materials so as to recover heat being lost in the exhaust gas stream. Examples of such an approach are disclosed in U.S. Pat. Nos. 3,726,697; 3,788,832; 3,880,639; and 3,953,190. Furthermore, these proposals return the sulfate material to the melting process, thereby saving on batch costs.
Unfortunately, most arrangements for contacting batch materials with exhaust gas have two major drawbacks: the batch must be agglomerated, and the particulate removal efficiency is dependent on bed size. Agglomerating the batch, usually by pelletizing or briquetting, is required in order to prevent the fine batch materials from being entrained by the exhaust gas stream. However, the cost of agglomerating the batch can substantially reduce and even exceed the economic gain from the heat recovery. Also, it has been found that the use of agglomerated batch is not always successful at avoiding dusting since abrasion in a moving bed at high temperatures can cause the loss of fine material from the surfaces of the agglomerates. When these fine materials are entrained in the exhaust gas stream, they contribute to the particulate emissions problem. Secondly, since beds of batch agglomerates remove particulates from the exhaust gas stream primarily by filtration, obtaining the desired degree of particulate removal can sometimes require undesirably large bed depths. A large bed depth is undesirable not only from the standpoint of equipment size, but more importantly, because of the accompanying high pressure drop which may require the use of additional blower means which likewise may substantially negate the economic advantages of heat recovery.
Preheating of loose glass batch material by contact with exhaust gases in fluidized beds has been proposed (e.g., U.S. Pat. No. 4,099,953). However, predicted separation of fine materials from the relatively coarse materials of the batch and entrainment of the fine materials in the exhaust gas stream have discouraged use of the fluidized bed approach to preheat glass batch.
In U.S. Pat. No. 3,753,743, there is disclosed a method of recovering waste heat from a glass furnace exhaust stream by passing the exhaust gas through a bed of cullet. The heated cullet is mixed with a caustic soda containing batch slurry in order to dry the slurry prior to feeding to the melting furnace. The patent includes no mention of particulate removal nor does it contemplate any use for heated cullet apart from the slurry drying function disclosed. It appears that little, if any, thermal benefit to the melting process is obtained by the method disclosed in the patent.
An arrangement is shown in U.S. Pat. No. 3,880,629 for returning collected particulates from a bag house to a glass melting furnace, but no attempt is made to recover waste heat.
The use of beds of electrostatically charged solids as the media for collecting particulates from gas streams is disclosed in U.S. Pat. Nos. 2,990,912; 4,126,435 and 4,144,359. None of these relates to glass melting processes nor do they deal with recovery of waste heat.